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Removed the sub box and speakers from my 2000 M Roadster tonight. It has the Harmon Kardon speaker system. Inspected the sub box and speakers and everything looks great. No seperation or damage to the speakers themselves. In fact the fabric on the speakers looks almost new. However. in Looking in the front of the speaker box I noted some cheap material that looks to be sound deadening material in the front of the box which was just floating freely within the box. Pulled it out through the sub gooseneck hole and removed it. Put the sub box back in and fired it up and it sounds great. I was getting some distortion which is why I took it apart in the first place. I believe that this material was possibly hitting on the speaker on occasion causing this distortion. Any problem leaving this out as it does not seem to affect the performance it a negative way with it out. Here is a picture. Any help from the experts is appreciated. Also put some vibration reducing pads on the speaker cover and with these small changes I can really crank up the volume and the bass with no distortion.

You can live without it. It seems to be an attempt to "soften" the bass sound to prevent popping, crackling, ans buzzing that can happen when large amounts of air are pushed around. More modern subwoofer designs use polyester fill (same as pillows) to slow down air movement and make the sub box acoustically larger. I removed my carpet-pad-like foam that you show and replaced mine with polyfill stuffing inside the entire subwoofer box. It really didn't make much difference at all. In fact, with 10 percent total harmonic distortion in the HK system, there is little you can do to make it worse.

You can live without it. It seems to be an attempt to "soften" the bass sound to prevent popping, crackling, ans buzzing that can happen when large amounts of air are pushed around. More modern subwoofer designs use polyester fill (same as pillows) to slow down air movement and make the sub box acoustically larger. I removed my carpet-pad-like foam that you show and replaced mine with polyfill stuffing inside the entire subwoofer box. It really didn't make much difference at all. In fact, with 10 percent total harmonic distortion in the HK system, there is little you can do to make it worse.

This. When I pulled my single sub I found the same material in it. It's really far too dense to have any effect at low frequencies. That style padding is more effective at eliminating standing waves inside the enclosure and only works for higher frequencies. When I built my home theater speakers I used egg-crate foam to absorb standing waves inside the enclosures.

When I installed my Kicker sub I used fiberglass insulation (same principle as polyfill) because I had some laying around. As Blacklane said, it bends physics a little bit and makes the enclosure perform as if it is slightly larger than it really is (perhaps 20%?), therefore lowering the overall resonant frequency of the box.